Stay sharp! Good quality pencils will allow the lead to be sharpened regularly. Needle sharp tips on your pencils are perfect for applying detail and getting the precision required to color intricate areas of a design. Thinner points will give you more control over the color produced, big fat rounded ends on your pencils make it difficult to get the brightest colors.
Always Color in a Consistent Direction. This is my first tip. It will make a bigger impact on the end result than anything else and any age can use it! The changes in direction, shown on the red kidney below, on the left side, are what make it look sloppy, even though I carefully stayed in the lines. Coloring in a consistent direction, as seen on the other kidney, is easier on the eyes and ends up looking very tidy!